Formation of a hemostatic plug, sealing an injured blood vessel wall, and initiation of a thrombotic lesion hinges upon the interaction of platelets with clotting factors and the vessel wall. These interactions are contgrolled by the availability of membrane receptors, and we plan to study the regulatory mechanisms governing the mobilization of receptors for fibrinogen and Factor VIIIVWF on human platelets such as prostacyclin-induced membrane changes preventing receptor availability. Since the hemostatic plug is impaired when the interaction of F. VIIIVWF, or the interaction of fibrinogen, with human platelets is defective, we will examine the cooperativity of F. VIIIVWF and fibrinogen in adhesion and aggregation of human platelets. Furthermore, we will investigate the relationship between platelet-derived clotting factors (F. VIIIVWF and fibrinogen) with their plasma counterparts in their interaction with platelet receptors. Our next line of inquiry will be focused on the assembly of clotting factors on platelet surfaces, namely the stepwise binding of F. VIIIVWF, F. VIIIC, and F. IXa. Finally, the proteolytic degradation of clotting factors, assembled on the surface of platelets, will be investigated as a potential mechanism for limiting the extent of platelet involvement in the assembly and activation of clotting factors on their surface. Several new methodological approaches to the problem of mobilization of receptors for clotting factors on human platelets will be introduced. Attempts will be made to identify platelet membrane component(s) controlling the availability of specific membrane receptors for fibrinogen and Factor VIIIVWF. The results of the proposed project will expand our knowledge of the fundamental regulatory mechanism(s) underlying the formation of a hemostatic plug and initiation of thrombosis, and thus, may provide better insight into the possibility of pharmacologic modulation of clotting factors--cell surfaces interaction.